#5556 (comment)

Seems like it would be better to not filter out client nodes in src/plugins/elasticsearch/lib/check_es_version.js. Although most of the time having old versions of client nodes will not break anything, client nodes do some preprocessing of the request, so there are no guarantees that old client nodes won't break things by preprocessing the request in a way that up-to-date versions of elasticsearch can't support.

Note that this is the only thing that the es_bool lib is used for.